Alkenes copper compound oxidations

Catalytic oxidations by copper compounds are mainly homolytic in nature. Copper salts have been extensively used in conjunction with molecular oxygen, peroxides and persulfate for the oxidation of a variety of alkenic and aromatic hydrocarbons.56,584... 

Alkenes can be oxidized to ketones of the same chain length by using salts of copper, palladium, and mercury as catalysts and air, electrolysis [120], hydrogen peroxide, or chromium compounds as oxidants [60, 65, 140, 565] (equation 90). 

Copper powder, copper bronze, copper(I) oxide, copper(II) oxide, copper(Il) sulfate, and cop-per(I) halides, typically applied as a suspension in refluxing solvent or alkene, are used extensively for intermolecular cyclopropanation with diazoacetic esters or diazomalonic esters, and for intramolecular cyclopropanation of unsaturated diazocarbonyl compounds. Bis(acetylacetonato)copper(Il) [Cu(acac)2], a more recently introduced catalyst, is only sparingly soluble in the typical solvents and alkenes which are used and is therefore applied under the same conditions. Catalysts such as trialkyl phosphite and triaryl phosphite complexes of copper(I) halides and salicylaldimatocopper(II) chelates [e.g. 1 (R = (R)-a-phenylethyl, R = /ert-butyl ) and 2 ] are soluble in many organic solvents and liquid alkenes. 

The cyclopropanation of gaseous alkenes, butadiene, and allene (see Section 1.2.1.2.4.2.6.3.3., Table 11, entry 1) by diazoacetic esters can be achieved by passing a vapor-gas mixture of the alkene and the diazo compound at atmospheric pressure through a tubular continuous flow reactor which contains a copper catalyst (ca. 10%) deposited on pumice. In this manner, alkyl cyclopropanecarboxylates were obtained in yields of up to 50% with cop-per(II) sulfate (typical reaction temperature 65-110"C, contact time 3.6 s) or copper(II) oxide (85-200°C, 5s) as catalysts. 

Copper compounds catalyze an exceedingly varied array of reactions, hetereogeneously, homogeneously, in the vapor phase, in organic solvents and in aqueous solutions. Many of these reactions, particularly if in aqueous solutions, involve oxidation-reduction systems and a Cu -Cu11 redox cycle. Molecular oxygen can often be utilized as oxidant, e.g., in copper-catalyzed oxidations of ascorbic acid and in the Wacker process (page 798) for conversion of alkenes into aldehydes. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-l,2-diols by acid-catalyzed hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with OSO4. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R2C , to give cyclopropanes. Nonhalo-genated cyclopropanes are best prepared by treatment of the alkene with CH2I2 and zinc-copper, a process called the Simmons-Smith reaction. 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Good yields of carbonyl compounds have also been obtained from the vapor-phase oxidation of alkenes by steam and air over palladium catalysts supported on charcoal.413 In this case, no copper cocatalyst is needed, presumably because palladium(II) is not reduced to palladium(O), but remains in the form of a stabilized palladium(Il) hydride which can react with 02 to give the hydroperoxidic species. 

Cyclic orthoesters derived from gem-diols offer a further route to alkenes. As part of a three-step conversion, they may be ring opened with hydrobromic acid to give O-acyl bromodeoxy compounds that undergo reductive elimination with copper-zinc. In this way, unsaturated nucleosides have been made by way of mixed 2y3,-bromo-2y3,-deoxy-3,/2, carboxyl-ates.174 A more direct route to alkenes from cyclic orthoesters involves heating in acetic anhydride together with zirconium oxide.175... 

Such J-mctals as Cu(I) [but not Cu(II)], form a variety of compounds with ethenes, for example [Cu(C2H4)(H20)2]C104 (from Cu, Cu2+, and C2H4) or Cu(C2H4)(bipy)+. It is necessary to mention that, of all the metals involved in biological systems, only copper reacts with ethylene [74b]. Such homoleptic alkene complexes can be useful intermediates for the synthesis of other complexes. The olefin complexes of the metals in high formal oxidation states are electron deficient and therefore inert toward electrophilic reagents. By contrast, the olefin complexes of the metals in low formal oxidation states are attacked by electrophiles such as protons at the electron-rich metal-carbon a-bonds [74c]. 

An unusual example of copper-catalyzed epoxidation using substoichiometric amounts of soluble Cu(ll) compounds in methylene chloride uses MCPBA as a terminal oxidant. As a mixture of cis- and /ra j -epoxides can be obtained from a single stereoisomeric alkene, a stepwise radical mechanism is suggested <2000TL1013>. 

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